1. Field of the Invention
The invention relates to the manufacturing of organic semiconductor layers formed of a mixture of a first and of a second semiconductor material, more specifically of organic semiconductor layers taking part in the forming of organic optoelectronic devices, especially of photodiodes.
2. Description of Related Art
Referring to FIG. 1, which is a simplified cross-section view of a prior art organic photodiode 10, such a photodiode usually comprises a stack formed, from bottom to top:                a transparent substrate 12, for example formed of a glass;        a layer 14 forming a first electrode, for example, formed of an indium-tin oxide, usually called “ITO”;        an injection layer 16, for example formed of a mixture of poly(3,4-ethylenedioxythiophene), commonly referred to as “PEDOT”, and of sodium poly(styrenesulfonate), commonly referred to as “PSS”, such a mixture being itself usually referred to as “PEDOT:PSS”. Injection layer 16 eases the passing of holes from first electrode 14 to active semiconductor layer 18;        an active semiconductor layer 18 forming a PN junction, formed of a mixture of two organic semiconductor materials P and N, usually a mixture of two polymers, for example, a mixture of poly(3-hexylthiophene), commonly referred to as “P3HT”, and of [6,6]-phenyl-C61-butyric acid methyl ester, commonly referred to as “DCBM”; and        a layer 20 forming a second electrode, for example, formed of calcium, of silver, or of aluminum, calcium being preferred due to its low work function which enables to collect electrons only.        
In operation, an electromagnetic radiation illuminates substrate 12 and the photons which reach active layer 18 create electron-hole pairs. By submitting electrodes 14 and 20 to a potential difference, a current is then collected, with a value depending on the illumination of photodiode 10.
The efficiency of such a photodiode 10 however depends on the contact surface area existing in active layer 18 between the P-type organic semiconductor material, for example, P3HT, and the N-type organic semiconductor material, for example, PCBM, the efficiency rapidly decreasing as the contact surface area decreases. Further, the efficiency also depends on the size of the domains, the latter having to be of small extension so that charges can cross them to reach the electrode without being recombined. On this regard, an ideal active layer 18 is thus formed of a mixture homogeneous at a molecular scale of the two organic semiconductor materials.
It is however not possible to obtain such a homogeneity with current deposition techniques. Indeed, active layer 18 is usually formed by depositing a solution comprising a solvent having the organic semiconductor materials dissolved or dispersed therein, and by then evaporating the solvent.
When the solvent evaporation rate is too small, a phase separation can then be observed, layer 18 being finally formed of a layer 22 of a first semiconductor material and of a layer 24 of a second semiconductor material. Contact surface 26 between the two materials is then very small and the efficiency of photodiode 10 is thus decreased.
As illustrated in FIGS. 2A-2D, this issue also arises when selecting a highly-volatile solvent, such as for example toluene. Thus, when the two semiconductor materials are mixed in toluene to form a mixture which is initially as homogeneous as possible, for example, by magnetic stirring (FIG. 2A), the obtained solution is very unstable. Local phase separations (FIG. 2B), of thermodynamic origin and due to physico-chemical attraction and repulsion phenomena between molecules, rapidly appear even before the solution is deposited on first electrode 14 (FIG. 2C). This phenomenon carries on during the evaporation of the volatile solvent and large areas formed of a single type of material can be finally observed in layer 18 (FIG. 2D).
Thus, it can usually be observed that 80% of the areas have at least one dimension greater than 10 μm. The total contact surface area between the two organic semiconductor materials is thus here again decreased, and the photodiode efficiency is low.
Document US-A-2009/050206 describes a method for manufacturing an organic semiconductor layer comprising a porous volume made of a first semiconductor material having its pores filled with a second semiconductor material. The porous volume is manufactured by using a phase separation of two materials mixed in a solution, and then, after the solution has solidified, the second material is removed to clear the lateral pores. First, with such a porous volume manufacturing method, it is difficult to accurately set the pore geometry, and especially their dimensions, and there further remain residues of the second material, which adversely affects the quality of the organic semiconductor layer.